Chapter 23

One-level storage system1

T. Kilbum / D. B. C. Edwards / M. J. LaniganF. H. Sumner

Summary After a brief survey of the basic Atlas machine, the paperdescribes an automatic system which in principle can be applied to anycombination of two storage systems so that the combination can be regarded

by the machine user as a single level. The actual system described relates

to a fast core store-drum combination. The effect of the system on instruc-

tion times is illustrated, and the tape transfer system is also introduced

since it fits basically in through the same hardware. The scheme incor-

porates a "learning" program, a technique which can be of greater impor-tance in future computers.

1. Introduction

In a universal high-speed digital computer it is necessary to have

a large-capacity fast-access main store. While more efficient oper-ation of the computer can be achieved by making this store all

of one type, this step is scarcely practical for the storage capacitiesnow being considered. For example, on Atlas it is possible toaddress 106 words in the main store. In practice on the first instal-

lation at Manchester University a total of 105 words are provided,

but though it is just technically feasible to make this in one level

it is much more economical to provide a core store (16,000 words)and drum (96,000 words) combination.

Atlas is a machine which operates its peripheral equipment on

a time division basis, the equipment "interrupting" the normal

main program when it requires attention. Organization of the

peripheral equipment is also done by program so that many pro-grams can be contained in the store of the machine at the same

time. This technique can also be extended to include several main

programs as well as the smaller subroutines used for controlling

peripherals. For these reasons as well as the fact that some orders

take a variable time depending on the exact numbers involved,it is not really feasible to "optimum" program transfers of infor-

mation between the two levels of store, i.e., core store and drum,in order to eliminate the long drum access time of 6 msec. Hence

a system has been devised to make the core drum store combi-

nation appear to the programmer as a single level of storage, the

l IRE Trans., EC-11, vol. 2, pp. 223-235, April, 1962.

requisite transfers of information taking place automatically. There

are a number of additional benefits derived from the scheme

adopted, which include relative addressing so that routines can

operate anywhere in the store, and a "lock out" facility to preventinterference between different programs simultaneously held in

the store.

2. The basic machine

The arrangement of the basic machine is shown in Fig. 1. Theavailable storage space is split into three sections; the private store

which is used solely for internal machine organization, the central

store which includes both core and drum store, in which all wordsare addressed and is the store available to the normal user, and

finally the tape store, which is the conventional backing-up large

capacity store of the machine. Both the private store and the main

core store are linked with the main accumulator, the B-store, and

the B-arithmetic unit. However the drum and tape stores only haveaccess to these latter sections of the machine via the main core

store.

The machine order code is of the single address type, and a

comprehensive range of basic functions are provided by normal

engineering methods. Also available to the programmer are a

number of extra functions termed "extracodes" which give auto-matic access to and subsequent return from a large number ofbuilt-in subroutines. These routines provide

1 A number of orders which would be expensive to providein the machine both in terms of equipment and also timebecause of the extra loading on certain circuits. An exampleof this is the order:

Shift accumulator contents ±n places where n is an integer.2 The more complex mathematical operations, e.g., sin x,

log x, etc.,

3 Control orders for peripheral equipments, card readers,

parallel printers, etc.,

4 Input-output conversion routines,

276

Chapter 23 One-level storage system 277

Operandaddress

Fixed store

2 meshesI « 4.096 words

Subsidiary store

1,024 words Hdecodeon digits

23,22,21

Core store

address fromcentrol

machine

Subsidiary store

address

8 tape decks

k0.5x10fi

words*

Main core store4 stocks

4 n 4,096 words

Drum store4 drums

k 24, 576 words

B store128 words24 digits

8 drithmeticunit

Peripherdl

eguipments

Mainaccumulator

Address channels—•- Information channels

(two way)

Fig. 1. Layout of basic machine.

5 Special programs concerned with storage allocation to

different programs being run simultaneously, monitoring

routines for fault finding and costing purposes, and the

detailed organization of drum and tape transfers.

All this information is permanently required and hence is kept

in part of the private store termed the "fixed store" [Kilburn and

Grimsdale, 1960a] which operates on a "read only" basis. This store

consists of a woven wire mesh into which a pattern of small

"linear" ferrite slugs are inserted to represent digital information.

The information content can only be changed manually and will

tend to differ only in detail between the different versions of the

Atlas computer. In Muse this store is arranged in two units each

of 4096 words, a unit consisting of 16 columns of 256 words, each

word being 50 bits. The access time to a word in any one column

is about 0.4 jusec. If a change of column address is required, this

figure increases by about 1 /usee due to switching transients in the

read amplifiers. Subsequent accesses in the new column revert to

0.4 jusec. The store operates in conjunction with a subsidiary core

store of 1024 words which provides working space for the fixed

store programs, and has a cycle time of about 1.8 jusec. There are

certain safeguards against a normal machine user gaining access

to addresses in either part of the private store, though in effect

he makes use of this store through the extracode facility.

The central store of the machine consists of a drum and core

store combination, which has a maximum addressable capacity of

about 106 words. In Muse the central store capacity is about 96,000

words contained on 4 drums. Any part of this store can be trans-

ferred in blocks of 512 words to/from the main core store, which

consists of four separate stacks, each stack having a capacity of

4096 words.

The tape system provides a very large capacity backing store

for the machine. The user can effect transfers of variable amounts

of information between this store and the central store. In actual

fact such transfers are organized by a fixed store program which

initiates automatic transfers of blocks of 512 words between the

tape store and the main core store. The system can handle eight

tape decks running simultaneously, each producing or demanding

a word on average every 88 jusec.The main core store address can thus be provided from either

the central machine, the drum, or the tape system. Since there

is no synchronization between these addresses, there has to be a

priority system to allocate addresses to the core store. The drum

has top priority since it delivers a word every 4 jusec, the tapenext priority since words can arise every 11 jusec from 8 decks

and the machine uses the core store for the rest of the available

time. A priority system necessarily takes time to establish its

priority, and so it has been arranged that it comes into effect only

at each drum or tape request. Thus the machine is not slowed

down in any way when no drum or tape transfers take place. The

effect of drum and tape transfers on machine speed is given in

Appendix 1.

To simplify the control commands given to the drum, tape, and

peripheral equipment in the machine, the orders all take the form

h->S or s->B and the identification of the required command

register is provided by the address S. This type of storage is clearly

widely scattered in the machine but is termed collectively the

V-store.

In the central machine the main accumulator contains a fast

adder [Kilburn et al., 1960b] and has built-in multiplication and

division facilities. It can deal with fixed or floating point numbers

and its operation is completely independent of the B-store and

B-arithmetic unit. The B-store is a fast core store (cycle time 0.7

jusec) of 120 twenty-four bit words operating in a word selected

partial flux switching mode [Edwards et al., I960]. Eight "fast"

B lines are also provided in the form of flip-flop registers. Of these,

three are used as control lines, termed main, extracode, and inter-

rupt controls respectively. The arrangement has the advantage

that the control numbers can be manipulated by the normal B-type

orders, and the existence of three controls permits the machine

to switch rapidly from one to another without having to transfer

control numbers to the core store. Main control is used when the

278 Part 3 The instruction-set processor level: variations in the processor Section 6 Processors with multiprogramming ability

central machine is obeying the current program, while the extra-

code control is concerned with the fixed store subroutines. The

interrupt control provides the means for handling numerous pe-

ripheral equipments which "interrupt" the machine when theyeither require or are providing information. The remaining "fast"

B lines are mainly used for organizational procedures, though B124

is the floating point accumulator exponent.

The operating speed of the machine is of the order of 0.5 X 106

instructions per second. This is achieved by the use of fast tran-

sistor logic circuitry, rapid access to storage locations, and an

extensive overlapping technique. The latter procedure is made

possible by the provision of a number of intermediate buffer stor-

age registers, separate access mechanisms to the individual units

of core store and parallel operation of the main accumulator and

B-arithmetic units. The word length throughout the machine is

48 bits which may be considered as two half-words of 24 bits each.

All store transfers between the central machine, the drum and tapestores are parity checked, there being a parity digit associated with

each half-word. In the case of transfers within the central store

(i.e., between main core store and drum) the parity digits associ-

ated with a given word are retained throughout the system. Tapetransfers are parity checked when information is transferred to

and from the main core store, and on the tape itself a check sum

technique involving the use of two closely spaced heads is used.

The form of the instruction, which allows for two B-modifica-

tions, and the allocation of the address digits is shown in Fig. 2a.

Half of the addressable store locations are allocated to the central

store which is identified by a zero in the most significant digit

of the address. (See Fig. 2b.) This address can be further subdivided

into block address, and line address in a block of 512 words. The

least significant digits, and 1, make it possible to address 6 bit

characters in a half word and digit 2 specifies the half word.

The function number is split into several sections, each section

relating to a particular set of operations, and these are listed in

Fig. 2c. The machine orders fall into two broad classes, and these

are

1 B codes: These involve operations between a B line specifiedby the BA digits in the instruction and a core store line

whose address can be modified by the contents of a B line

determined by the Bm digits. There are a total of 128 B

lines, one of which, B , always contains zero. Of the otherlines 90 are available to the machine user, 7 are special

registers previously mentioned, and a further 30 are used

by extracode orders.

2 A codes: These involve operations between the Accumulatorand a core store line whose address can now be doubly

Function

10 bits

3. One-level store concept

The choice of system for the fast access store in a large scale

computer is governed by a number of conflicting factors which

include speed and size requirements, economic and technical

difficulties. Previously the problem has been resolved in two ex-

treme cases either by the provision of a very large core store, e.g.,the 2.5 megabit [Papian, 1957] store at M.I.T., or by the use of

a small core store (40,000 bits) expanded to 640,000 bits by a drum

store as in the Ferranti Mercury [Lonsdale and Warburton, 1956;

Kilburn et al., 1956] computer. Each of these methods has its

disadvantages, in the first case, that of expense, and in the second

case, that of inconvenience to the user, who is obliged to programtransfers of information between the two types of store and this

can be time consuming. In some instances it is possible for an

expert machine user to arrange his program so that the amount

of time lost by the transfers in the two-level storage arrangementis not significant, but this sort of "optimum" programming is not

very desirable. Suitable interpretative coding [Brooker, 1960] can

permit the two-level system to appear as one level. The effect is,

however, accompanied by an effective loss of machine speed

which, in some programs and depending on details of machine

design, can be quite severe, varying typically, for example, be-

tween one and three.

The two-level storage scheme has obvious economic advan-

tages, and inconvenience to the machine user can be eliminated

by making the transfer arrangements completely automatic. In

Atlas a completely automatic system has been provided with tech-

niques for minimizing the transfer times. In this way the core

and drum are merged into an apparent single level of storage with

good performance and at moderate cost. Some details of this ar-

rangement on the Muse are now provided.The central store is subdivided into blocks of 512 words as

shown by the address arrangements in Fig. lb. The main core store

is also partitioned into blocks of this size which for identification

purposes are called pages. Associated with each of these core store

page positions is a "page address register" (P.A.R.) which contains

the address of the block of information at present occupying that

page position. When access to any word in the central store is

required the digits of the demanded block address are comparedwith the contents of all the page address registers. If an "equiva-lence" indication is obtained then access to that particular page

position is permitted. Since a block can occupy any one of the

32 page positions in the core store it is necessary to modify some

digits of the demanded block address to conform with the page

positions in which an equivalence was obtained.

Chapter 23 One-level storage system 279

These processes are necessarily time consuming but by provid-

ing a by-pass of this procedure for instruction accesses (since, in

general, instruction loops are all contained in the same block) then

most of this time can be overlapped with a useful portion of the

machine or core store rhythm. In this way information in the core

store is available to the machine at the full speed of the core store

and only rarely is the over-all machine speed affected by delaysin the equivalence circuitry.

If a "not equivalence" indication is obtained when the de-

manded block address is compared with the contents of the

P.A.R.'s then that address, which may have been B-modified, isfirst stored in a register which can be accessed as a line of the

V-store. This permits the central machine easy access to this ad-

dress. An "interrupt" also occurs which switches operation of the

machine over to the interrupt control, which first determines the

cause of the interrupt and then, in this instance, enters a fixed

store routine to organize the necessary transfers of information

between drum and core store.

A. Drum transfers

On each drum, one track is used to identify absolute block posi-tions around the drum periphery. The records on these tracks are

read into the registers which can be accessed as lines of the

V-store and this permits the present angular drum position to be

determined, though only in units of one block. In this way the

time needed to transfer any block while reading from the drums

can be assessed. This time varies between 2 and 14 msec since

the drum revolution time is 12 msec and the actual transfer time

2 msec.

The time of a writing transfer to the drums has been reduced

by writing the block of information to the first available emptyblock position on any drum. Thus the access time of the drum

can be eliminated provided there are a reasonable number of

empty blocks on the drum. This means, however, that transfers

to/from the drum have to be carried out by reference to a direc-

tory and this is stored in the subsidiary store and up-dated when-

ever a transfer occurs.

When the drum transfer routine is entered the first action isto determine the absolute position on a drum of the required block.

The order is then given to carry out the transfer to an empty page

position in the core store. The transfer occurs automatically as

soon as the drum reaches the correct angular position. The pageaddress register in the vacant position in the core store is set to

a^ specific block number for drum transfers. This technique sim-

plifies the engineering with regard to the provision of this number

280 Part 3 The instruction-set processor level: variations in the processor Section 6 Processors with multiprogramming ability

from the drum and also provides a safeguard against transferringto the wrong block.

As soon as the order asking for a read transfer from the drum

has been given the machine continues with the drum transfer

program. It is now concerned with determining a block to be

transferred back from the core store to the drum. This is necessary

to ensure an empty core store page position when the next read

transfer is required. The block in the core store to be transferred

has to be carefully chosen to minimize the number of transfers

in the program and this optimization process is carried out by a

learning program, details of which are given in Sec. 5. The opera-tion of this program is assisted by the provision of the "use" digitswhich are associated with each page position of the core store.

To interchange information between the core store and drums,

two transfers, a read from and a write to the drum are necessary.These have to be done sequentially but could occur in either order.

The technique of having a vacant page position in the core store

permits a read transfer to occur first and thus allows the time for

the learning program to be overlapped either into the waiting

period for the read transfer or into the transfer time itself. In the

time remaining after completion of the learning program an entryis made into the over-all supervisor program for the machine, and

a decision is taken concerning what the machine is to do until

the drum transfer is completed. This might involve a change to

a different main program.A program could ask for access to information in a page position

while a drum or tape transfer is taking place to that page. This

is prevented in Atlas by the use of a "lock out" (L.O.) digit which

is provided with each Page Address Register. When a lock out

digit is set at 1, access to that page is only permitted when the

address has been provided either by the drum system, the tape

system, or the interrupt control. The latter case permits all trans-

fers from paper tape, punched card, and other peripheral equip-

ments, to be handled without interference from the main program.When the transfer of a block has been completed the organizingprogram resets the L.O. digit to zero and access to that page

position can then be made from the central machine. It is clear

that the L.O. digit can also be used to prevent interference be-

tween programs when several different ones are being held in the

machine at the same time.

In Sec. 3 it was stated that addresses demanding access to the

core store could arise from three distinct sources, the central

machine, the drum, and the tape. These accesses are complicated

because of (1) the equivalence technique, and (2) the lock out digit.

The various cases and the action that takes place are summarized

in Table 1.

The provision of the Page Address Registers, the equivalence

circuitry, and the learning program have permitted the core store

and drum to be regarded by the ordinary machine user as a one-

level store, and the system has the additional feature of "floating

address" operation, i.e., any block of information can be stored

in any absolute position in either core or drum store. The minimum

access time to information in this store is obviously limited by

the core store and its arrangement and this is now discussed.

B. Core store arrangement

The core store is split into four stacks, each with individual address

decoding and read and write mechanisms. The stacks are then

combined in such a way that common channels into the machine

for the address, read and write digits are time shared between

the various stacks. Sequential address positions occur in two stacks

alternately and a page position which contains a block of 512

sequential addresses is thus arranged across two stacks. In this wayit is possible to read a pair of instructions from consecutive ad-

dresses in parallel by increasing the size of the read channel. This

permits two instructions to be completely obeyed in three store

"accesses." The choice of this particular storage arrangement is

discussed in Appendix 2.

The coordination of these four stacks is done by the "core stack

coordinator" and some features of this are now discussed, startingwith the operation of a single stack.

Table 1 Comparison of demanded block address with contents of the P.A.R.'s resultant state of equivalence and lock out circuits

Source of address

( Equivalence 1

(Lock out = 0)[E.Q.]

Not equivalence

[N.E.Q.]

( Equivalence

\ Lock out

ice 1

= 1)[E.Q. 6- L.O.]

1. Central Machine

2. Drum System3. Tape System

Access to required page position

Access to required page position

Access to required page position

Enter drum transfer routine

Fault condition indicated

Fault condition indicated

Not available to this programFault condition indicated

Fault condition indicated

Chapter 23 One-level storage system 281

C. Operation of a single stack of core store

The storage system employed is a coincident current M.I.T. system

arranged to give parallel read out of 50 digits. The reading opera-

tion is destructive and each read phase of the stack cycle is fol-

lowed by a write phase during which the information read out

may be rewritten. This is achieved by a set of digit staticizors

which are loaded during the read phase and are used to control

the inhibit current drivers during the write phase. When new

information is to be written into the store a similar sequence is

followed, except that the digit staticizors are loaded with the new

information during the read phase. A diagram indicating the

different types of stack cycle is shown in Fig. 3.

Stack

request

Read

phase

Readstrobe

Write

phase

"^T

+=H-i r

i.0)

,ck—

I

ruest I—I

Stack

reqi

Readphase

Write

strobe

Write

phase

ISr

(*)

,Ck —1 i-uest

|I

Stack

req

Readphase

Readstrobe

Writestrobe

Write

phase

Ui_r

(c)

TA = access time; Tc = cyclic time; Wo - wait for address decodingand loading of address register; W w - wait for release of write holdup.

Fig. 3. Basic types of stack cycle, (a) Read order (s-

(a—> s). (c) Read-write order (b + s —» S).

A), (b) Write order

There is a small delay WD (~100 m/isec) between the "stackrequest" signal, Sfi, and the start of the read phase to allow for

setting of the address state and the address decoding. The outputinformation from the store appears in the read strobe period, which

is towards the end of the read phase. In general, the write phasestarts as soon as the read phase ends. However, the start of the

write phase may be held up until the new information is available

from the central machine. This delay is shown as Ww in Fig. 3c.The interval TA between the stack request and the read strobe

is termed the stack access time, and in practice this is approxi-

mately one third of the cycle time Tc . Both TA and Tc are functions

of the storage system and assuming that Ww is zero have typicalvalues of 0.7 jusec and 1.9 jusec respectively. A holdup gate in the

request channel prevents the next stack request occurring before

the end of the preceding write phase.

D. Operation of the main core store with the central machine

A schematic diagram of the essentials of the main core store con-trol system is shown in Fig. 4. The control signals SA t and SA2indicate whether the address presented is that of a single word

or a pair of sequentially addressed instructions. Assuming that the

flip-flop F is in the reset condition, either of these signals results

in the loading of the buffer address register (B.A.R.). This loading

is done by the signal B.A.B.A. which also indicates that the buffer

register in the central machine has become free.

In dealing with the first request the block address digits in the

B.A.R. are compared with the contents of all the page address

registers. Then one of the indications summarized in Table 1 and

indicated in Fig. 4 is obtained. Assuming access to the requiredstore stack is permitted then a set C.S.F. signal is given which

resets the flip-flop F. If this occurs before the next access request

arises, then the speed of the system is not store-limited. In most

cases SET CSF is generated when the equivalence operation on

the demanded block address is complete, and the read phase of

the appropriate stack (or stacks) has started. Until this time the

information held in the B.A.R. must not be allowed to change.In Fig. 5 a flow diagram is shown for the various cases which can

arise in practice.

When a single address request is accepted it is necessary toobtain an "equivalence" indication and form the page location

digits before the stack request can be generated. The SET CSF

signal then occurs as soon as the read phase starts. If a "not equiva-

lent" or "equivalent and locked out" indication is obtained a stack

request is not generated, and the contents of the B.A.R. are copiedin to a line of the V-store before SET CSF is generated.

When access to a pair of addresses is requested (i.e., an instruc-

282 Part 3 The instruction-set processor level: variations in the processor Section 6 Processors with multiprogramming ability

Buffer address registerI

Block oddress |Line address

Page address regO|

[Page address reg 1

Not instructionoddress

|Poge oddress reg 31 1

Equivalence

circuitry,Pogedigits

~j j rEQ NEQ EQaiO

sr.r

CSP

Instructionaddress

Page digitregister

Comparisoncircuit

Right

page

Wrong

page

Control circuitry

Stackrequest

Stockaddress

Stack

Chapter 23 One-level storage system 283

3 It is necessary to ensure a certain minimum time between

successive read strobes from the core store stacks to allow

satisfactory operation of the parity circuits, which take

about 0.4 |iisec to check the information. This time could

be reduced, but as it is only possible to get such a condition

for a small part of the normal instruction timing cycle it

was not thought to be an economical proposition.

The basic machine timing is now discussed.

4. Instruction times

In high-speed computers, one of the main factors limiting speed

of operation is the store cycle time. Here a number of techniques,

e.g., splitting the core store into four separate stacks and extracting

two instructions in a single cycle, have been adopted despite a

fast basic cycle time of 2 jusec in order to alleviate this situation.

The time taken to complete an instruction is dependent upon

1 The type of instruction (which is defined by the function

digits)

2 The exact location of the instruction and operand in the

core or fixed store since this can affect the access time

3 Whether or not the operand address is to be modified

4 In the case of floating point accumulator orders, the actual

numbers themselves

5 Whether drum and/or tape transfers are taking place

The approximate times for various instructions are given in

Table 2. These figures relate to the times between completinginstructions when a long sequence of the same type of instruction

is obeyed. While this method is not ideal, it is necessary because

in practice obeying one instruction is overlapped in time with

some part of three other instructions. This makes the detailed

timing complicated, and so the timing sequence is developed

slowly by first considering instructions obeyed one after another.

It is convenient to make these instructions a sequence of floating

point additions with both instruction and operand in the core store

and with the operand address single B-modified.

To obey this instruction the central machine makes two re-

quests to the core store, one for the instruction and the second

for the operand. After the instruction is received in the machine

the function part has to be decoded and the operand address

modified by the contents of one of the B registers before the

operand request can be made. Finally, after the operand has been

obtained the actual accumulator addition takes place to complete

the instruction. The time from beginning to end of one instruction

is 6.05 jusec and an approximate timing schedule is as follows in

Table 3.

If no other action is permitted in the time required to complete

the instruction (steps 1 to 8 in Table 3), then the different sections

of the machine are being used very inefficiently, e.g., the accumu-

lator adder is only used for less than 1.1 jusec. However, the orga-

nization of the computer is such that the different sections such

as store stacks, accumulator and B-arithmetic unit, can operate

Table 2 Approximate instruction times

Type of instruction

284 Part 3|

The instruction-set processor level: variations in the processor Section 6|

Processors with multiprogramming ability

Table 3f Timing sequence for floating point addition (instructionsand operands in the core store)

Chapter 23 One-level storage system 285

Copy

j

to|

Accumulotor busyocc

Operands,cck

tre

1f

e5t| Equivalence [ Read

OperandStart second of pair

(Function! g modification '^T^I decode I

Copy|

to Lace

Accumulator busy_

|Equivolence

Stack

request

Start

next pair

I

Instruction

request ifci

I III

Stack

request

Equivolence[Function!I decode I B modification

CopyIto Locc

Operandrequest

i

Acumulator busy_ J

Stack

request

Equivolence

Start secondof pair

IFunctionl

I decode I B modification

Startnext pair

i

Instruction

request

|'o | Equivolence

Fig. 6. Timing diagram for a sequence of floating point addition orders. (Single-address modification.)

1 Element of first vector into accumulator. (Operand B-modi-

fied.)

2 Multiply accumulator by element of second vector. (Oper-and B-modified.)

3 Add partial product to accumulator.

4 Copy accumulator to store line containing partial product.

5 Alter count to select next elements and repeat.

The time for this loop with instructions and operands on the

core store is 12.2 jusec. The value of the overlapping techniqueis shown by the fact that the time from starting the first instruction

to finishing the second is approximately 10 jusec.

When the drum or tape systems are transferring informationto or from the core store then the rate of obeying instructions

which also use the core store will be affected. The affect is dis-

cussed in more detail in Appendix 1. The degree of slowing down

is dependent upon the time at which a drum or tape request occurs

relative to machine requests. It also depends on the stacks used

by the drum or tape and those being used by the central machine.

The approximate slowing down is by a factor of 25 per cent duringa drum transfer and by 2 per cent for each active tape channel.

(See Appendix 1.)

5. The drum transfer learning program

The organization of drum transfers has been described in Sec. 2A.

After the transfer of the required block from the drum to the core

store has been initiated, the organizing program examines the state

of the core store, and if empty pages still exist, no further action

is taken. However, if the core store is full it is necessary to arrangefor an empty page to be made available for use at the next non-

equivalence. The selection of the page to be transferred could be

made at random; this could easily result in many additional trans-fers occurring, as the page selected could be one of those in current

use or one required in the near future. The ideal selection, which

would minimize the total number of transfers, could only be made

by the programmer. To make this ideal selection the programmerwould have to know (1) precisely how his program operated, which

is not always the case, and (2) the precise amount of core store

available to his program at any instant. This latter information

is not generally available as the core store could be shared by other

central machine programs, and almost certainly by some fixed store

program organizing the input and output of information from slow

peripheral equipments. The amount of core store required by this

fixed store program is continuously varying [Kilburn et al., 1961].The only way the ideal pattern of transfers can be approachedis for the transfer program to monitor the behavior of the main

program and in so doing attempt to select the correct pages to

be transferred to the drum. The techniques used for monitoringare subject to the condition that they must not slow down the

operation of the program to such an extent that they offset anyreduction in the number of transfers required. The method de-

scribed occupies less than 1 per cent of the operating time, and

the reduction in the number of transfers is more than sufficient

to cover this.

286 Part 3 The instruction-set processor level: variations in the processor Section 6|

Processors with multiprogramming ability

That part of the transfer program which organizes the selection

of the page to be transferred has been called the "learning" pro-

gram. In order for this program to have some data on which to

operate, the machine has been designed to supply information

about the use made of the different pages of the core store bythe program being monitored.

With each page of the core store there is associated a "use"

digit which is set to "1" whenever any line in that page is accessed.

The 32 "use" digits exist in two lines of the V-store and can be

read by the learning program, the reading automatically resettingthem to zero. The frequency with which these digits are read is

governed by a clock which measures not real time but the number

of instructions obeyed in the operation of the main program. This

clock causes the learning program to copy the "use" digits to a

list in the subsidiary store every 1024 instructions. The use of an

instruction counter rather than a normal clock to measure "time"

for the learning program is due to the fact that the operationsof the main program may be interrupted at random for random

lengths of time by the operation of peripheral equipments. With

an instruction counter the temporal pattern of the blocks used

will be the same on successive runs through the same part of the

program. This is essential if the learning program is to make use

of this pattern to minimize the number of transfers.

When a nonequivalence occurs and after the transfer of the

required block has been arranged, the learning program again adds

the current values of the "use" digits to the list and then uses

this list to bring up to date two sets of times also kept in the

subsidiary store. These sets consist of 32 values of t and T, one

of each for each page of the core store. The value of t is the lengthof time since the block in that page has been used. The value of

T is the length of the last period of inactivity of this block. The

accuracy of the values of t and T is governed by the frequencywith which the "use" digits are inspected.

The page to be written to the drum is selected by the appli-cation in turn of three simple tests to the values of t and T.

1 Any page for which t > T + 1, or

2 That page with t =£ and (T—

t) max, or

3 That page with Tmax (all t = 0).

The first rule selects any page which has been currently out

of use for longer than its last period of inactivity. Such a pagehas probably ceased to be used by the program and is therefore

an ideal one to be transferred to the drum. The second rule ignoresall pages with t = as they are in current use, and then selectsthe one which, if the pattern of use is maintained, will not be

required by the program for the longest time. If the first two rules

fail to select a page the third ensures that if the page finally

selected is wrong, in that it is immediately required again, then,

as in this case, T will become zero and the same mistake will not

be repeated.For all the blocks on the drum a list of values of t is kept.

The values of t are set when the block is transferred to the drum:

t = time of transfer—value of t for transferred pageWhen a block is transferred to the core store the value of t isused to set the value of T.

T = time of transfer—value of t for this block= length of last period of inactivity

For the block transferred from the drum t is set to 0.

In order to make its decision the learning program has onlyto update two short lists and apply at the most three simple rules;

this can easily be done during the 2 msec transfer time of the block

required as a result of the nonequivalence. As the learning program

uses only fixed and subsidiary store addresses it is not slowed down

during the period of the drum transfer.

The over-all efficiency of the learning program cannot be

known until the complete Atlas system is working. However, the

value of the method used has been investigated by simulating the

behavior of the one-level store and learning program on the

Mercury computer at Manchester University. This has been done

for several problems using varying amounts of store in excess of

the core store available. One of these was the problem of formingthe product A of two 80th order matrices B and C. The threematrices were stored row by row each one extending over 14

blocks, only 14 pages of core store were assumed to be available.

The method of multiplication was

fcn X 1st row of C = partial answer to 1st row of Ab12 X 2nd row of C + partial answer = second partial answer,

etc.

Thus matrix B was scanned once, matrix C 80 times and each row

of matrix A 80 times.Several machine users were asked to spend a short time writing

a program to organize the transfers for a general matrix multipli-

cation problem. In no case when the method was applied to the

above problem were fewer than 357 transfers required. A programwritten specifically for this problem which paid great attention

to the distribution of the rows of the matrices relative to block

divisions required 234 transfers. The learning program required274 transfers; the gain over the human programmer was chiefly

Chapter 23 One-level storage system 287

due to the fact that the learning program could take full advantage

of the occasions when the rows of A existed entirely within one

block.

Many other problems involving cyclic running of single or

multiple sets of data were simulated, and in no case did the learn-

ing program require more transfers than an experienced human

programmer.

A. Prediction of drum transfers

Although the learning program tends to reduce the number of

transfers required to a minimum, the transfers which do occur still

interrupt the operation of the program for from 2 to 14 msec as

they are initiated by nonequivalence interrupts. Some or all of

this time loss could be avoided by organizing the transfers in

advance. A very experienced programmer having sole use of thecore store could arrange his own transfers in such a way that no

unnecessary ones ever occurred and no time was ever wasted

waiting for transfers to be completed. This would require a greatdeal of effort and would only be worthwhile for a program that

was going to occupy the machine for a long time. By using the

data accumulated by the learning program it is possible to recog-nize simple patterns in the use made by a program of the various

blocks of the one-level store. In this way a prediction programcould forecast the blocks required in the near future and organizethe transfers. By recording the success or failure of these forecasts

the program could be made self-improving. For the matrix multi-

plication problem discussed above the pattern of use of the blocks

containing matrix C is repeated 80 times, and a considerable

degree of success could be obtained with a simple prediction

program.

6. Conclusions

A specific system for making a core-drum store combination appearas a single level store has been described. While this is the actual

system being built for the Atlas machine the principles involved

are applicable to combinations of other types of store. For exam-

ple, a tunnel diode-fast core store combination for an even faster

machine. An alternative which was considered for Atlas, but which

was not as attractive economically, was a fast core-slow core store

combination. The system too can be extended to three levels of

storage, and indeed if 106 words of total storage had to be provided

then it would be most economical to provide it on a third level

of store such as a file drum.

The automatic system does require additional equipment and

introduces some complexity, since it is necessary to overlap the

time taken for address comparison into the store and machine

operating time if it is not to introduce any extra time delays.Simulated tests have shown that the organization of drum transfers

are reasonably efficient and other advantages which accrue, such

as efficient allocation of core storage between different programsand store lock out facilities are also invaluable. No matter how

intelligent a programmer may be he can never know how many

programs or peripheral equipments are in operation when his

program is running. The advantage of the automatic system is that

it takes into account the state of the machine as it exists at any

particular time. Furthermore if as in normal use there is some sort

of regular machine rhythm even through several programs, there

is the possibility of making some sort of prediction with regardto the transfers necessary. This involves no more hardware and

will be done by program. However, this stage will probably be left

until results on the actual system are obtained.

It can be seen that the system is both useful and flexible in

that it can be modified or extended in the manner previouslyindicated. Thus despite the increase in equipment, the advantageswhich are derived completely justify the building of this automatic

system.

APPENDIX 1 ORGANIZATION OF THE ACCESS REQUESTSTO THE CORE STORE

There are three sources of access requests to the core store, namelythe central machine, the drum, and the tape systems. In decidinghow the sequence of requests from all three sources are to beserialized and placed in some sort of order, a number of facts have

to be considered. These are

1 All three sources are asynchronous in nature.

2 The drum and tape systems can make requests at a fairly

high rate compared with the store cycle time of approxi-

mately 2 jusec. For example, the drum provides a request

every 4 jusec and the tape system every 11 /tsec when all8 channels are operative.

3 The drum and tape systems can only be stopped in multiplesof a block length, i.e., 512 words. This means that any systemdevised for accessing the core store must deal with both

the average rates of drum and tape requests specified in 2.

Only the central machine can tolerate requests being stoppedat any time and for any length of time. From these facts a

request priority can be stated which is

a Drum request.b Tape request.c Central machine request.

288 Part 3 The instruction-set processor level: variations in the processor

4 A machine request can be accepted by the core store, butbecause there is no place available to accept the core store

information, its cycle is inhibited and further requests held

up. In the case of successive division orders this time can

be as long as 20 ^usec, in which case 5 drum requests could

be made. To avoid having an excessive amount of buffer

storage for the drum two techniques are possible:a When drums or tapes are operative do not permit ma-

chine requests to be accepted until there is a placeavailable to put the information.

b Store the machine request and then permit a drum or

tape request.

The latter scheme has been adopted because it can be

accommodated more conveniently and it saves a small

amount of time.

5 If the central machine is using the private store then it is

desirable for drum and tape transfers to the core store not

to interfere with or slow down the central machine in anyway.

6 When the central machine, drum and tape are sharing thecore store then the loss of central machine speed should

be roughly proportional to the activity of the drum or tape

systems. This means that drum or tape requests must"break" into the normal machine request channel as and

when required.

The system which accommodates all these points is now dis-

cussed. Whenever a drum or tape request occurs inhibit signalsare applied to request channel into the core stack coordinator and

also to the stack request channels from this coordinator. This

results in a "freezing" of the state of flip-flop F (Fig. 5) and this

state is then inspected (Fig. 7, point X). If the state is "busy" this

means that a machine order has been stopped somewhere between

the loading of the buffer address register (B.A.R.) and the stack

request. Normally this time interval can vary from about 0.5 /isec

if there are no stack request holdups, to 20 jusec in the case of

certain accumulator holdups. In either case sufficient time is al-

lowed after the inspection to ensure that the equivalence operationhas been completed. If an equivalence indication is obtained all

the information relevant to this machine order (i.e., the line ad-

dress, page digits, stack(s) required and type of stack order) are

stored for future reference. Use is made here of the page digit

register provided to allow the by-pass on the equivalence circuitry

for instruction accesses. The core store is then made free for access

by the drum or the tape. If the core store had been found to be

free on inspection, the above procedure is omitted.

F flip-flop frozen

y Inspect state of* F flip-flop

1

Busy

Wait for

equivalence

completed

I

Store machine order

I

Free F flip-flop

Drum tope accessto core store -Drum/tape priority

-

Remove stack request

Inhibit signals

Stock requestfor drum /tape

Orum/tape request

Is there a storedmachine order ?

Perm it stack request___f^\nhibits to reapply W

Allow to proceed(if possible)

Stack request ofstored machine order

Apply inhibits tostack request channelsand to machine requestchannels (if these arenot already applied)

Hos the stack requestof a stored machineorder been stopped 7

rNo 7es

Remove inhibitson machine requestchannels

Fig. 7. Drum and tape break in systems.

A drum or tape access (as decided by the priority circuit) to

the core store then occurs, which removes the inhibits on the stack

request channels. When the stack request for the drum or tapecycle is initiated these inhibits are allowed to reapply. At this stage

(Fig. 7, point Y), if there is a stored machine order it is allowed

to proceed if possible. The inhibits on the machine request chan-

nels are removed when the stack request for the stored machine

order occurs. If there is no stored machine order this is done

Chapter 23 One-level storage system 289

immediately, and the central machine is again allowed access to

the core store. However, another drum or tape request can arise

before the stack request of the stored machine order occurs, in

particular because this latter order may still be held up by the

central machine. If this is the case the drum or tape is allowed

immediate access and a further attempt is made to complete the

stored machine order when this drum or tape stack request occurs.

If the stored machine order was for an operand, the content

of the page digit register will correspond to the location of this

operand. The next machine request for an instruction pair will

then almost certainly result in a "wrong page" indication. This

is prevented by arranging that the next instruction pair access does

not by-pass the equivalence circuitry.

The effect on the machine speed when the drum or tapes are

transferring information to or from the core store is dependent

upon two factors. First, upon the proportion of time during which

the buffer register in the core coordinator is busy dealing with

machine requests, and secondly, upon the particular stacks beingused by the central machine and the drum or tape. If the computeris obeying a program with instructions and operands on the fixed

or subsidiary store then the rate of obeying instructions is un-

affected by drum or tape transfers. A drum or tape interruptoccurring when the B.A.R. is free prevents any machine address

being accepted onto this buffer for 1.0 /usee. However, if the B.A.R.

is busy then the next machine request to the core store is delayed

until 1.8 /usee after the interrupt if different stacks are being used,

or until 3.4 /usee after the interrupt if the stacks are the same.

When the machine is obeying a program with instructions and

operands on the core store the slowing down during drum transfers

can be by a factor of two if instructions, operands, and drum

requests use the same stacks. It is also possible for the machine

to be unaffected. The effect on a particular sequence of orders

can be seen by considering the one discussed in Sec. 4 and illus-

trated in Fig. 6. In this sequence the instructions are on stacks

and 1 while the operands are on stacks 2 and 3. If the drum

or tape is transferring alternately to stacks and 1 then the effect

of any interrupt within the 3.2 /usee of an instruction pair is to

increase this time by between 0.5 and 3.4 /usee depending uponwhere the interrupt occurred. The average increase is 1.8 /useeand for a tape transfer with interrupts every 88 /usee the computercan obey instructions at 98 per cent of the normal rate. Duringdrum transfers the interrupts occur every 4 jusec which would

suggest a slowing down to 60 per cent of normal. However, for

any regular sequence of orders the requests to the core store bythe machine and by the drum rapidly become synchronized with

the result in this particular case that the machine can still operate

at 80 per cent of its normal speed.

APPENDIX 2 METHODS OF DIVISION OF THE MAINCORE STORE

The maximum frequency with which requests can be dealt with

by a single stack core store is governed by the cycle time of the

store. If the store is divided into several stacks which can be cycled

independently then the limit imposed on the speed of the machine

by the core store is reduced. The degree of division which is chosen

is dependent upon the ratio of core store cycle time to other

machine operations and also upon the cost of the multiple selec-

tion mechanisms required.

Considering a sequence of orders in which both the instruction

and operand are in the core store, then for a single stack store

the limit imposed on the operating speed by the store is two cycletimes per order, i.e., 4 /usee in Atlas. This is significantly larger

than the limits imposed by other sections of the computer

(Sec. 4). If the store is divided into two stacks and instructions and

operands are separated, then the limit is reduced to 2 /usee which

is still rather high. The provision of two stacks permits the ad-

dressing of the store to be arranged so that successive addresses

are in alternate stacks. It is therefore possible by making requeststo both stacks at the same time to read two instructions together,so reducing the number of access times to three per instruction

pair. Unfortunately such an arrangement of the store means that

operands are always on the same stacks as instruction pairs, and

the limit imposed by the cycle time is still 2 /usee per order even

if the two operand requests in the instruction pair are to different

stacks and occur at the same time.

Division into any number of stacks with the addressing system

working through each stack in turn cannot reduce the limit below

2 /usee since successive instructions normally occur in successive

addresses and are therefore in the same stack. However, four stacks

arranged in two pairs reduces the limit to 1 /usee as the operandscan always be arranged to be on different stacks from the instruc-

tion pairs. In order to reduce the limit to 0.5 /usee it is necessary

to have eight stacks arranged in two sets of four and to read four

instructions at once, which would increase the complexity of the

central machine.

The limit of 1 /usee is quite sufficient and further division with

the stacks arranged in pairs only enables the limit to be more easilyobtained by suitable location of the instructions and operands.

The location of instructions and operands within the core store

is under the control of the drum transfer program; thus when there

290 Part 3 The instruction-set processor level: variations in the processor Section 6|

Processors with multiprogramming ability

Chapter 10

One-Level Storage System^

routines can operate anywhere in the store, and a "lock out"

facihty to prevent interference between different programssimultaneously held in the store.

T. Kilbuni / D. B. G. Edwards / M. J. Lanigan /F. H. Sumner

Summary Aiter a brief survey of the basic Atlas machine, the paperdescribes an automatic system which in principle can be applied to anycombination of two storage systems so that the combination can be

regarded by the machine user as a single level. The actual systemdescribed relates to a fast core store-drum combination. The effect of the

system on instruction times is illustrated, and the tape transfer system is

also introduced since it fits basically in through the same hardware. The

scheme incorporates a "learning" program, a technique which can be of

greater importance in fiiture computers.

1. Introduction

In a universal high-speed digital computer it is necessary to have a

large-capacity fast-access main store. While more efficient opera-tion of the computer can be achieved by making this store all ofone type, this step is scarcely practical for the storage capacitiesnow being considered. For example, on Atlas it is possible toaddress 10* words in the main store. In practice on the first

installation at Manchester University a total of 10^ words are

provided, but though it is just technically feasible to make this inone level it is much more economical to provide a core store(16,000 words) and drum (96,000 words) combination.

Atlas is a machine which operates its peripheral equipment on atime division basis, the equipment "interrupting" the normalmain program when it requires attention. Organization of the

peripheral equipment is also done by program so that manyprograms can be contained in the store ofthe machine at the sametime. This technique can also be extended to include several main

programs as well as the smaller subroutines used for controlling

peripherals. For these reasons as well as the fact that some orderstake a variable time depending on the exact numbers involved, itis not really feasible to "optimum" program transfers of informa-tion between the two levels of store, i.e., core store and drum, in

order to eliminate the long drum access time of 6 msec. Hence a

system has been devised to make the core drum store combination

apjjear to the programmer as a single level of storage, the

requisite transfers of information taking place automatically.There are a number of additional benefits derived from thescheme adopted, which include relative addressing so that

2. The Basic Machine

The arrangement of the basic machine is shown in Fig. 1. Theavailable storage space is split into three sections; the private store

which is used solely for internal machine organization, the central

store which includes both core and drum store, in which all wordsare addressed and is the store available to the normal user, and

finally the tape store, which is the conventional backing-up large

capacity store of the machine. Both the private store and the main

core store are linked with the main accumulator, the B-store, andthe B-arithmetic unit. However the drum and tape stores onlyhave access to these latter sections of the machine via the maincore store.

The machine order code is of the single address type, and a

comprehensive range of basic fimctions are provided by normal

engineering methods. Also available to the programmer are anumber of extra functions termed "extracodes" which giveautomatic access to and subsequent return from a large number ofbuilt-in subroutines. These routines provide

1 A number of orders which would be expensive to provide inthe machine both in terms of equipment and also timebecause of the extra loading on certain circuits. An exampleof this is the order:

Shift accumulator contents ±n places where n is an integer.

Operandoddress

Eitrocodecontrol

FneO storo2 meshes

» 4.096 wofd!

HSubsidiary store LjJ1,024 »ordi n

; ^

decodeon digits

23.22,21

Core store

address fromcentral

machine

Address trom

Subsidiary store

address

h

Core stora

addr«ts

Topt store8 tope decks

U 5x10^ wordsapproximate

Main core jtor*4 jtocks

4 « 4,096 words

Drum store4 drums

x24,576«wrds

8 Store126 words24 digrrs Peripheral

equipments

Main

accumulator

—" Address chofinols-"•- Informotion channels

(tw«o woy)

'IRE Trans., EC-11, vol. 2, April 1962, pp. 223-235 Fig. 1. Layout of basic machine.

135

136 Part 1 Fundamentals Section 3 | Computers of Historical Significance

2 The more complex mathematical operations, e.g. , sin .t, logX, etc.

3 Control orders for peripheral equipments, card readers,

parallel printers, etc.

4 Input-output conversion routines.

5 Special programs concerned with storage allocation to

different programs being run simultaneously, monitoringroutines for fault finding and costing purposes, and the

detailed organization of drum and tape transfers.

All this information is permanently required and hence is keptin part of the private store termed the "fixed store" [Kilbum and

Grimsdale, 1960] which operates on a "read only" basis. This

store consists of a woven wire mesh into which a pattern of small

"linear" ferrite slugs are inserted to represent digital information.

The information content can only be changed manually and will

tend to differ only in detail between the different versions of the

Atlas computer. In Muse this store is arranged in two units each of4096 words, a unit consisting of 16 columns of 256 words, each

word being 50 bits. The access time to a word in any one column is

about 0.4 n,sec. If a change of column address is required, this

figure increases by about 1 jtsec due to switching transients in the

read amplifiers. Subsequent accesses in the new column revert to0.9 jxsec. The store operates in conjunction with a subsidiary core

store of 1024 words which provides working space for the fixed

store programs, and has a cycle time of about 1.8 jtsec. There are

certain safeguards against a normal machine user gaining access to

addresses in either part of the private store, though in effect he

makes use of this store through the extracode facility.The central store of the machine consists of a drum and core

store combination, which has a maximum addressable capacity ofabout 10' words. In Muse the central store capacity is about

96,000 words contained on 4 drums. Any part of this store can betransferred in blocks of 512 words to/from the main core store,which consists of four separate stacks, each stack having a capacityof 4096 words.

The tape system provides a very large capacity backing store forthe machine. The user can effect transfers of variable amounts ofinformation between this store and the central store. In actual fact

such transfers are organized by a fixed store program which

initiates automatic transfers of blocks of 512 words between the

tape store and the main core store. The system can handle eighttape decks running simultaneously, each producing or demandinga word on average every 88 |xsec.The main core store address can thus be provided from either

the central machine, the drum, or the tape system. Since there is

no synchronization between these addresses, there has to be a

priority system to allocate addresses to the core store. The drumhas top priority since it delivers a word every 4 p,sec, the tape next

priority since words can arise every 1 1 jjisec from 8 decks and the

machine uses the core store for the rest of the available time. A

priority system necessarily takes time to establish its priority, and

so it has been arranged that it comes into effect only at each drumor tape request. Thus the machine is not slowed dovm in any waywhen no drum or tape transfers take place. The effect ofdrum and

tape transfers on machine speed is given in Appendix 1.

To simplify the control commands given to the drum, tape, and

peripheral equipment in the machine, the orders all take the form

b —* S or s —* B and the identification of the required command

register is provided by the address S. This type of storage is

clearly widely scattered in the machine but is termed collectivelythe V-store.

In the central machine the main accumulator contains a fast

adder [Kilbum, et al., 1960fo] and has built-in multiplication and

division facilities. It can deal with fixed or floating point numbers

and its operation is completely independent of the B-store and

B-arithmetic unit. The B-store is a fast core store (cycle time 0.7

|xsec) of 120 twenty-four bit words operating in a word selected

partial flux switching mode [Edwards et al., I960]. Eight "fast" Blines are also provided in the form of flip-flop registers. Of these,three are used as control lines, termed main, extracode, and

interrupt controls respectively. The arrangement has the advan-

tage that the control numbers can be manipulated by the normal

B-type orders, and the existence of three controls permits the

machine to switch rapidly from one to another without having to

transfer control numbers to the core store. Main control is used

when the central machine is obeying the current program, whilethe extracode control is concerned with the fixed store subrou-

tines. The interrupt control provides the means for handlingnumerous peripheral equipments which "interrupt" the machine

when they either require or are providing information. The

remaining "fast" B lines are mainly used for organizational

procedures, though B124 is the floating point accumulator

exponent.

The operating speed of the machine is of the order of 0.5 x 10*

instructions per second. This is achieved by the use of fast

transistor logic circuitry, rapid access to storage locations, and an

extensive overlapping technique. The latter procedure is made

possible by the provision of a number of intermediate buffer

storage registers, separate access mechanisms to the individual

units of core store and parallel operation of the main accumulator

and B-arithmetic units. The word length throughout the machine

is 48 bits which may be considered as two half-words of 24 bits

each. All store transfers between the central machine, the drum

and tape stores are parity checked, there being a parity digitassociated with each half-word. In the case of transfers within the

central store (i. e. , between main core store and drum) the parity

digits associated with a given word are retained throughout the

system. Tape transfers are parity checked when information is

Chapter 10{

One-Level Storage System 137

transferred to and from the main core store, and on the tape itself

a check sum technique involving the use of two closely spacedheads is used.

The form of the instruction, which allows for two B-

modifications, and the allocation of the address digits is shown in

Fig. 2a. Half of the addressable store locations are allocated to the

central store which is identified by a zero in the most significant

digit of the address. (See Fig. 2b.) This address can be fiirther

subdivided into block address and line address in a block of 512

words. The least significant digits, and 1, make it possible to

address 6 bit characters in a half word and digit 2 specifies the half

word.

The function number is split into several sections, each section

relating to a particular set of operations, and these are listed in

Fig. 2c. The machine orders fall into two broad classes, and these

B codes: These involve operations between a B line

specified by the Ba digits in the instruction and a core storeline whose address can be modified by the contents of a Bline determined by the Bm digits. There are a total of 128 B

lines, one of which. Bo, always contains zero. Of the otherlines 90 are available to the machine user, 7 are special

registers previously mentioned, and a further 30 are used

by extracode orders.

A Codes: These involve operations between the Accumula-tor and a core store line whose address can now be doublymodified first by contents of B^ and then by the contents of

Ba- Both fixed and floating point orders are provided, and

in the latter case numbers take the form of X8'', the digitallocation of X and Y being shown in Fig. 2d. When fixedpoint working occurs, use is made only of the X digits.

3. One-Level Store Concept

The choice of system for the fast access store in a large scale

computer is governed by a number of conflicting factors which

include speed and size requirements, economic and technical

difficulties. Previously the problem has been resolved in two

extreme cases either by the provision of a very large core store,

e.g., the 2.5 megabit [Papian, 1957] store at M.I.T., or by the use

of a small core store (40,000 bits) expanded to 640,000 bits by a

drum store as in the Ferranti Mercury [Lonsdale and Warburton,1956; Kilbum et al. , 1956] computer. Each of these methods has

its disadvantages, in the first case, that of expense, and in the

second case, that of inconvenience to the user, who is obliged to

program transfers of information between the two types of store

and this can be time consuming. In some instances it is possiblefor an expert machine user to arrange his program so that the

amount of time lost by the transfers in the two-level storage

Function

(0 bits

138 Part 1 Fundamentals Section 3 I Computers of Historical Significance

details of machine design, can be quite severe, varying typically,for example, between one and three.

The two-level storage scheme has obvious economic advantag-es, and inconvenience to the machine user can be eliminated by

making the transfer arrangements completely automatic. In Atlas

a completely automatic system has been provided with techniquesfor minimizing the transfer times. In this way the core and drum

are merged into an apparent single level of storage with good

performance and at moderate cost. Some details of this arrange-ment on the Muse are now provided.The central store is subdivided into blocks of 512 words as

shown by the address arrangements in Fig. 2b. The main core

store is also partitioned into blocks of this size which for

identification purposes are called pages. Associated with each of

these core store page positions is a "page address register"

(P.A.R.) which contains the address of the block of information at

present occupying that page position. When access to any word inthe central store is required, the digits of the demanded block

address are compared with the contents of all the page address

registers. Ifan "equivalence" indication is obtained, then access to

that particular page position is permitted. Since a block can

occupy any one of the 32 page positions in the core store, it is

necessary to modify some digits of the demanded block address to

conform with the page positions in which an equivalence was

obtained.

These processes are necessarily time consuming but by provid-

ing a by-pass of this procedure for instruction accesses (since, in

general, instruction loops are all contained in the same block) then

most of this time can be overlapped with a useful portion of the

machine or core store rhythm. In this way information in the core

store is available to the machine at the full speed of the core store

and only rarely is the over-all machine speed aflFected by delays in

the equivalence circuitry.

If a "not equivalence" indication is obtained when the demand-ed block address is compared with the contents of the P.A.R.'s,then that address, which may have been B-modified, is first storedin a register which can be accessed as a line of the V-store. This

permits the central machine easy access to this address. An

"interrupt" also occurs which switches operation of the machine

over to the interrupt control, which first determines the cause of

the interrupt and then, in this instance, enters a fixed store

routine to organize the necessary transfers of information between

drum and core store.

A. Drum Transfers

On each drum, one track is used to identify absolute block

positions around the drum periphery. The records on these tracksare read into the registers which can be accessed as lines of the

V-store and this permits the present angular drum position to be

determined, though only in units of one block. In this way the

time needed to transfer any block while reading from the drums

can be assessed. This time varies between 2 and 14 msec since the

drum revolution time is 12 msec and the actual transfer time 2

msec.

The time ofa writing transfer to the drums has been reduced by

writing the block of information to the first available empty block

position on any drum. Thus the access time of the drum can beeliminated provided there are a reasonable number of emptyblocks on the drum. This means, however, that transfers to/from

the drum have to be carried out by reference to a directory andthis is stored in the subsidiary store and up-dated whenever a

transfer occurs.

When the drum transfer routine is entered the first action is todetermine the absolute position on a drum of the required block.The order is then given to carry out the transfer to an empty page

position in the core store. The transfer occurs automatically as

soon as the drum reaches the correct angular position. The pageaddress register in the vacant position in the core store is set to a

specific block number for drum transfers. This technique simpli-fies the engineering with regard to the provision of this number

from the drum and also provides a safeguard against transferringto the wrong block.

As soon as the order asking for a read transfer from the drum

has been given, the machine continues with the drum transfer

program. It is now concerned with determining a block to be

transferred back from the core store to the drum. This is necessaryto ensure an empty core store page position when the next read

transfer is required. The block in the core store to be transferred

has to be carefully chosen to minimize the number of transfers in

the program and this optimization process is carried out by a

learning program, details of which are given in Sec. 5. The

operation of this program is assisted by the provision of the "use"

digits which are associated with each page position of the core

store.

To interchange information between the core store and drums,

two transfers, a read from and a write to the drum, are necessary.These have to be done sequentially but could occur in either

order. The technique of having a vacant page position in the core

store permits a read transfer to occur first and thus allows the time

for the learning program to be overlapped either into the waiting

period for the read transfer or into the transfer time itself In the

time remaining after completion of the learning program an entryis made into the over-all supervisor program for the machine, and

a decision is taken concerning what the machine is to do until the

drum transfer is completed. This might involve a change to a

different main program.A program could ask for access to information in a page position

while a drum or tape transfer is taking place to that page. This is

prevented in Atlas by the use of a "lock out" (L.O.) digit which is

provided with each Page Address Register. When a lock out digitis set at 1, access to that page is permitted only when the address

has been provided either by the drum system, the tape system, or

Chapter 10|

One-Level Storage System 139

the interrupt control. The last case permits all transfers from

paper tape, punched card, and other peripheral equipments, to

be handled without interference from the main program. Whenthe transfer ofa block has been completed, the organizing program

resets the L.O. digit to zero and access to that page position can

then be made from the central machine. It is clear that the L.O.

digit can also be used to prevent interference between programswhen several different ones are being held in the machine at the

same time.

In Sec. 3 it was stated that addresses demanding access to the

core store could arise from three distinct sources, the central

machine, the drum, and the tape. These accesses are complicatedbecause of (1) the equivalence technique, and (2) the lock out

digit. The various cases and the action that takes place are

summarized in Table I.

The provision of the Page Address Registers, the equivalence

circuitry', and the learning program have permitted the core store

and drum to be regarded by the ordinary machine user as a

one-level store, and the system has the additional feature of

"floating address" operation, i.e., any block of information can be

stored in any absolute position in either core or drum store. Theminimum access time to information in this store is obviouslylimited by the core store and its arrangement, and this is now

discussed.

B. Core Store Arrangement

The core store is split into four stacks, each with individual

address decoding and read and write mechanisms. The stacks are

then combined in such a way that common channels into themachine for the address, read and write digits, are time shared

between the various stacks. Sequential address positions occur in

two stacks alternately and a page position which contains a block

of 512 sequential addresses is thus arranged across two stacks. In

this way it is possible to read a pair of instructions from

consecutive addresses in parallel by increasing the size of the read

channel. This permits two instructions to be completely obeyed in

three store "accesses." The choice of this particular storage

arrangement is discussed in Appendix 2.

The coordination of these four stacks is done by the "core stack

coordinator" and some features of this are now discussed, startingwith the operation of a single stack.

C. Operation ofa Single Stack of Core Store

The storage system employed is a coincident current M.I.T.

system arranged to give parallel read out of 50 digits. The reading

operation is destructive and each read phase of the stack cycle is

followed by a write phase during which the information read out

may be rewritten. This is achieved by a set of digit staticizers

which are loaded during the read phase and are used to control

the inhibit current drivers during the write phase. When newinformation is to be written into the store, a similar sequence is

followed, except that the digit staticizors are loaded with the new

information during the read phase. A diagram indicating thedifferent types of stack cycle is shown in Fig. 3.

There is a small delay W^ (=100 usee) between the "stackrequest" signal, Sfl, and the start of the read phase to allow for

setting of the address state and the address decoding. The outputinformation from the store appears in the read strobe period,which is towards the end of the read phase. In general, the write

phase starts as soon as the read phase ends. However, the start of

the write phase may be held up until the new information isavailable from the central machine. This delay is shown as Wj,. in

Fig. 3c. The interval Ta between the stack request and the read

strobe is termed the stack access time, and in practice this is

approximately one-third of the cycle time Tc- Both Ta and Tc are

functions of the storage system and assuming that W„ is zero have

typical values of 0.7 jjLsec and 1.9 |xsec respectively. A holdup gatein the request channel prevents the next stack request occurring

before the end of the preceding write phase.

D. Operation of the Main Core Store

with the Centra] Machine

A schematic diagram of the essentials of the main core storecontrol system is shown in Fig. 4. The control signals SA, and SAjindicate whether the address presented is that of a single word or

a pair of sequentially addressed instructions. Assuming that the

flip-flop F is in the reset condition, either of these signals resultsin the loading of the bufier address register (B.A.R.). This loading

is done by the signal B.A. B. A. which also indicates that the bufifer

register in the central machine has become free.

In dealing with the first request the block address digits in the

B.A.R. are compared with the contents of all the page address

registers. Then one of the indications summarized in Table 1 and

Table 1 Comparison of Demanded Block Address with Contents of the P.A.R.'s Resultant State of Equivalence and Lock

Out Circuits

Source of address

{Equivalence

[Lock out =

[E.Q.]

Not equivalence\N.E.Q.]

{Equivalence 1

{Lock out = i I

\E.Q.i^L.O.]

1. Central Machine2. Drum System3 Tape System

Access to required page positionAccess to required page positionAccess to required page position

Enter drum transfer routineFault condition Indicated

Fault condition indicated

Not available to this programFault condition indicated

Fault condition Indicated

140 Part 1 Fundamentals Section 3|Computers of Historical Significance

Chapter 10|

One-Level Storage System 141

SA1 OR SA2

Woit for

core store

free

Sir>gle

LoodBAR.

Won foregu< valence

ond formation

of page digits

Woit isee text)

Woil for

equivalenceond formation

of poge digits

Eqiiivolence

Not equivolent

or equivolentond locked

Waif (see text]

Copy BAR. Stackto t^ line request

Start reod

phote

SET CSF SET CSF SET CSF

Fig. 5. Flow diagram of main core store controi.

system. The assumption will normally be true, except when

crossing block boundaries. The latter cases are detected andcorrected by comparing the true position page digits obtained as a

result of the equivalence operation with the contents of the page

digit register, and a "right page" or "wrong page" indication isobtained. (See Fig. 4.) If a wrong page is accessed this is indicatedto the central machine and the read out is inhibited. The true pagelocation digits are copied into the page digit register, so that the

required instruction pair will be obtained when next requested.The read out to the central machine is also inhibited for "not

equivalent" or "equivalent and locked out" indications.

In Fig. 5 the waiting time indicated immediately before the

stack request is generated can arise for a number of reasons:

I The preceding write phase of that stackfinished.

las not yet

2 The central machine is not yet ready either to acceptinformation from the store or to supply information to it.

3 It is necessary to ensure a certain minimum time betweensuccessive read strobes from the core stacks to allow

satisfactory operation of the parity circuits, which take

about 0.4 n,sec to check the information. This time could be

reduced, but as it is only possible to get such a condition for

a small part of the normal instruction timing cycle it was not

thought to be an economical proposition.

The basic machine timing is now discussed.

4. Instruction Times

In high-speed computers, one of the main factors limiting speed of

operation is the store cycle time. Here a number of techniques,e.g., splitting the core store into four separate stacks and

extracting two instructions in a single cycle, have been adopted

despite a fast basic cycle time of 2 jisec in order to alleviate this

situation. The time taken to complete an instruction is dependentupon

1 The type of instruction (which is defined by the function

digits)

2 The exact location of the instruction and operand in thecore or fixed store since this can aflFect the access time

3 Whether or not the operand address is to be modified

4 In the case of floating point accumulator orders, the actual

numbers themselves

3 Whether drum and/or tape transfers are taking place

The approximate times for various instructions are given inTable 2. These figures relate to the times between completinginstructions when a long sequence of the same type of instructionis obeyed. While this method is not ideal, it is necessary because

in practice obeying one instruction is overlapped in time with

some part of three other instructions. This makes the detailed

timing complicated, and so the timing sequence is developed

slowly by first considering instructions obeyed one after another.

It is convenient to make these instructions a sequence of floatingpoint additions with both instruction and operand in the core store

and with the operand address single B-modified.

To obey this instruction the central machine makes two

requests to the core store, one for the instruction and the second

for the operand. After the instruction is received in the machine

the function part has to be decoded and the operand address

modified by the contents of one of the B registers before the

operand request can be